keyword-model: when book id given, sort first those results and look for more

Instead of providing only results of the book id being specified, sort those
first and then add results from other books.

This is in order to start the removal of the [Current|All books] filter buttons.
keyword-model: minor coding style fixes

Always declare variables at the beginning of the context.
keyword-model: always prefer exact link in the current book
sidebar: remove [Current][All Books] filter buttons

Search is always done in all books, sorting current ones first.

https://bugzilla.gnome.org/show_bug.cgi?id=707608
keyword-model: keep track of what the current book was when search was done
sidebar: hits for current book are given in bold
keyword-model: prefer specifying current book with the 'book:' prefix

E.g. looking for "book:glib something" will sort first the hits in book 'glib'.
sidebar: remove unused variable
window: avoid using deprecated stock icons